Scientists theorize that human women frequently searched for Neanderthals

New York. Humans and Neanderthals occasionally came into contact when they lived in the same areas tens of thousands of years ago. But we don’t know much about who was involved with whom or why.

A new genetic analysis provides some old rumors: matings were more frequent between human women and Neanderthal men.

How exactly this happened remains a big unknown. Did human women venture into Neanderthal populations or were Neanderthal men drawn to larger human enclaves? Were these interactions peaceful, confusing, secret, or even violent?

“I don’t know if we’ll ever get a definitive answer about how this happened, since we can’t travel back in time,” said population genetics expert Xinjun Zhang of the University of Michigan, commenting on the new analysis.

But the study, published Thursday in the journal Scienceshows “that whenever Neanderthals and modern humans have mated, there has been a preference for male Neanderthals and female modern humans, rather than the other way around,” said author Alexander Platt, who studies genetics at the University of Pennsylvania.

Scientists know that Neanderthals and humans mated because there is a small but important percentage of Neanderthal DNA in most modern humans outside of sub-Saharan Africa, including genes that can help us fight some diseases and make us more susceptible to others.

But they also know that Neanderthal DNA is not evenly distributed throughout the human genome.

In particular, there is a striking lack of Neanderthal DNA on the human X chromosome, one of the sets of genes in each cell known as the sex chromosome, compared to the amount of Neanderthal DNA on the cell’s other non-sex chromosomes.

Scientists thought that perhaps the genes in those areas were simply not beneficial, or even harmful. Maybe people with those genetic patterns didn’t survive as well, so evolution filtered them out over time.

Or perhaps, they thought, the difference could be explained by how the two species mixed.

To try to solve the puzzle, Platt and his colleagues analyzed the Neanderthal genome and human DNA that intermingled during a “mating event” 250,000 years ago.

By comparing these genes, they found a more human fingerprint on the Neanderthal X chromosome, the same chromosome that, in humans, has less Neanderthal DNA than would be expected.

The most likely explanation for this mirror image pattern is mating behavior. This is due to the way sex chromosomes are passed from parents to children, Platt explained. Since genetic females have two X chromosomes and genetic males have one X and one Y chromosome, on average, two out of every three X chromosomes in a population are inherited from mothers.

If more human females mated with Neanderthal males than vice versa, over thousands of years you would expect to see exactly what they found: more human DNA on Neanderthal X chromosomes and less Neanderthal DNA on human X chromosomes.

“I think they’ve made some really important steps toward filling in the missing pieces of the puzzle,” said Joshua Akey, who studies evolutionary genomics at Princeton University and was not involved in the new study.